Method of reclaiming bituminous road material



Patented May 29, 1934 UNITED STATES ATNT METHOD OF RECLAIMING BITUMINOUS ROAD MATERIAL Norman Henry Taylor, Singapore, Straits Settlements 4 Claims.

This invention relates to asphalt paving and more particularly to the reclaiming of asphalt paving mixtures which have become unserviceable owing to faulty design, excessive trafiic, unstable foundations or from any other cause.

The process hereinafter described is particularly applicable to steam-rolled asphalt paving but is also applicable to rock and mastic asphalts.

Steam-rolled asphalt paving consists normally of (a) an upper sand sheet asphalt surface laid on a binder or base course composed, for example, of approximately seventy-five per cent. of stone, twenty per cent. of sand and five per cent. of bitumen, which adheres firmly to the upper sheet asphalt surface so that the two cannot be separated when the worn paving mixture is taken up from the road, or (b) a stone-filled asphalt mixture, being a sand sheet asphalt mixture to which a maximum of twenty-five per cent. of stone is added as an adulterant, laid either with or without a binder course, or (c) an asphalt concrete mixture containing from fifty to seventy-five per cent. of stone.

The most modern practice is to employ such mixtures that the compressed pavement contains not less than two and not more than five per cent. of voids so that with a given mineral aggregate the bitumen content must be very exactly controlled.

I am aware that old asphalt paving has been reused by pulverizing the old material in a pulverizer of the impact type, mixing the pulverized material with approximately half its weight of sand, heating the mixture, adding additional bitumen and filler and mixing the product. The addition of a large quantity of sand is necessary to enable the mixture to pass through the type of drum normally used in preparing new mixtures without sticking and considerable quantities of bitiunen and filler have to be added to the hot mixture leaving the drum in order to obtain a satisfactory paving material. The addition of large quantities of new material considerably reduces the economy of the reclamation process as the full value of the old material cannot be realized unless sufiicient paving area and funds are available for utilizing the surplus.

The type of drum normally used for heating sand and stone in the process for preparing new asphalt mixtures carries projections on its inner surface which pick up the material to be heated, carry it to the top of the drum and allow it to fall through the hot gases which are made to pass through the drum to the chimney.

A drum of this type is satisfactory so long as the material passing through the drum does not contain suflicient bitumen to render it adhesive when hot. As the heating drums normally revolve at a speed of approximately five revolutions per. minute, the material to be heated remains in contact with the projections for several seconds before falling and since the temperature of the drum is always considerably higher than that to which asphaltic materials can be heated without being oxidized, there is always a great risk of the material suffering from local overheating. If the hot material fails to fall away cleanly from the projections a deposit remains which rapidly oxidizes, adheres firmly to the drum and forms a base for further deposits. In this way a layer of hard oxidized material is built up inside the drum and may have, at the end of a days run, a thickness equal to the projection and is not only exceedingly difiicult to remove but enormously reduces the heating efficiency of the drum.

If all the projections are removed so as to leave a plain drum, it is found that mixtures carrying a much higher proportion of bitumen will pass through the drum without any layer being formed but will adhere long enough to the surface to be efliciently agitated by the revolving drum and be given a motion down the incline. In riveted drums the formation of a layer of asphaltic material may commence from the projection formed by the rivet heads so that countersunk rivets should be used wherever possible.

The relative richness of various mixtures can best be defined by the extent to which the bitumen fills the voids in the mineral matter in a compressed sample of the material and it is found that, notwithstanding the actual weight of bitumen in the mixture, a mixture which, when compressed ata temperature of 300 F. under a compression of 5,000 pounds per square inch, contains seven and a half per cent. or more of voids, will pass through a plain drum without sticking 01', forming a layer of oxidized material on the inner surface.

The proportion of mineral matter which must be added to the pulverized old material to increase the voids to a minimum of seven and a half per cent. will vary with the type of mixture being treated for in mixtures which contain stone an increase in voids takes place during pulverizing owing to the re-distribution of stone sizes as a result of crushing.

The proportion of added mineral matter will also vary considerably with the size of the added material as the addition of sand will increase the voids much more rapidly than the addition of crushed stone as will be seen from the test results given below.

Proportion in parts by 100 parts weight oi added sand old material Percent 14. 8

Percent Percent 9. 28

Percent 12. 85

Percent Voids in compressed Perce mixture 1. 75

Proportion in parts by weight of added in. Stone Percent Per- Voids in compressed Percent cent mixture 1. 75 2. 19

Percent 3. 09

Percent 4. 50

Percent 4. 01

It will be seen that quite a small proportion of additional sand will raise the voids in the material to be treated to the required limit and that the amount can readily be ascertained by experiment.

The rate of feeding and the furnace temperature should be so controlled that the heated material leaves the drum at a temperature of not less than 300 F. and not more than 350 F. On leaving the drum the mixture should pass into a hopper prior to being weighed out in batches for mixing with the required quantities of bitumen or flux oil and filler.

Since the material to be treated will vary considerably and may consist of sheet asphalt or asphalt concrete with adhering layers of binder, it is not possible to specify the final product in terms of percentage of bitumen and the granularmetric analysis of the mineral matter. The final product will, however, be satisfactory if the voids in and the resistance to deformation of a compressed sample of the mixture are within certain limits.

If a sample of the final product is compressed, at a temperature of 300 F. in a suitable mould under a compression of 5,000 pounds per square inch, this being the consolidating pressure normally obtained when the mixture is laid on the road, the percentage of voids in the compressed sample should be within the following limits.

Mixtures containing 50 or over by weight of material retained on a The permissible limits for mixtures containing less than 50% of material retained on a 10 mesh sieve can be interpolated between the limits given.

The lower limit is to ensure that the pavement will not become slippery after being subjected for some time to trafiic while the upper limit is to ensure that the pavement will not absorb moisture to a harmful extent.

It is essential that the resistance to deformation of the paving mixture should be such that while no rutting or waving will occur under trafiic, the pavement will not be so rigid as to be unable to follow slight movements of the subgrade without cracking.

This is ensured if the resistance to deformation of the paving mixture is approximately between 1,500 and 3,000 pounds, the said resistance being expressed by the load in pounds which, when applied at a uniform rate over an area two inches in diameter centrally situated on the upper surface of a disc four and a half inches in diameter and one and a half inches thick, the said disc having been formed of the material under examination at a temperature of 300 F. and under a pressure of 5,000 pounds per square inch, cooled, and afterwards maintained at a temperature approximating to the maximum temperature experienced on sun exposed surfaces in the locality in which the mixture is to be laid for a period of one hour prior to testing, shall cause the peripheral ring of unloaded material to fracture.

Since the material leaving the drum has, when compressed, seven and a half per cent. or more of voids, it is possible by adding a bitumen and 'a filler with suitable characteristics, to satisfy the conditions given above and at the same time counteract the hardening which invariably takes place after a mixture has been exposed for some years to the sun. The proportions of bitumen and filler to add in any particular case must be ascertained by experiment for, while additional bitumen and filler both tend to reduce the voids, filler tends to increase and bitumen tends to reduce the resistance to deformation.

Example No. 1

The old mixture to be reclaimed consisted of Trinidad sheet asphalt on binder, each layer being one and a half inches thick.

A typical analysis of this material after passing through the pulverizer was as follows:-

The specific gravity of a heated and compressed sample of this material was 2.228 indicating voids of 6.05%.

Tests on bitumen recovered from the mixture gave the following results:-

Penetration (100 grms. 5 secs. 77 F.) 8 Melting point (ring and ball) 158 F. Ductility at 77 F 6 cms.

Fine sand of the following grading was added Per cent Passing 200 mesh sieve Nil Passing 80 mesh sieve 65.0 Passing 40 mesh sieve 35. 0

in the proportion of ten parts of old material to one part of sand by weight.

After passing through the heating drum the mixture was combined with limestone filler and flux oil in the following proportions:-

The flux oil and filler were added cold.

A disc of the finished mixture heated and compressed at 5,000 lbs. per sq. inch showed:-

Specific gravity Voids 2.59% Resistance to deformation 2,100 lbs.

Tests on bitumen recovered from the mixture gave the following results:-

Penetration (100 grms. 5 secs. 77 F.) 45 Melting point (ring and ball) 135 F. Ductility at 77 F 70 cms.

Example No. 2

This mixture had failed owing to there being an excess of bitumen resulting in a low resistance to deformation. A typical analysis of the mixture after it has passed through the pulverizer was as follows:

The specific gravity of a heated and compressed sample of this material was 2.32 indicating voids of 1.23

Tests on bitumen recovered from the mixture gave the following results:

Penetration (100 grins. 5 secs. 77 F.) 36 Melting point (ring and ball) 127 F. Ductility at 77 F 100+ Fine sand similar to that used in the previous example was added in the proportion of seven and a half parts of old material to one part of sand.

After passing through the heating drum the mixture was combined with limestone filler and bitumen of penetration of 25 in the following proportions:--

Pounds Bitumen 5 Limestone filler 25 Hot material from heating drum 500 The bitumen was added at a temperature of 350 F. and the filler was added cold.

A disc of the finished mixture, heated and compressed at 5,000 lbs. per sq. inch, showed:-

Specific gravity 2.30 Voids 2.94% Resistance to deformation 1.850 lbs.

Tests on bitumen recovered from the mixture gave the following results:--

Penetration (100 grms. 5 secs. 77 F.) 30 Melting point (ring and ball) 137 F. Ductility at 77 F 100+ Having now described my invention, what I claim as new and desire to secure by Letters Patent is:

1. Process of reconditioning used asphalt paving material consisting in increasing the voids in a heated and compressed sample of the pulverized material to a minimum of seven and a half per cent. by the addition of mineral material,

heating the mixture in a heated inclined revolving drum devoid of projections on its interior, and maintaining the temperature of the drum so that the heated material leaves the drum at a temperature of not less than 300 F. and not more than 350 F.

2. Process of reconditioning used asphalt paving material consisting in increasingthe voids in a heated and compressed sample of the pulverized material to a minimum of seven and a half per cent. by the addition of mineral material, heating the mixture in a heated inclined revolving drum devoid of projections on its interior, maintaining the temperature of the drum so that the heated material leaves the drum at a temperature of not less than 300 F. and not more than 350 F. and conveying the hot combustion gases which heat the exterior of the drum into direct contact with the material under treatment.

3. Process of reconditioning used asphalt paving material consisting in increasing the voids in a heated and compressed sample of the pulverized material to a minimum of seven and a half per cent. by the addition of mineral material, heating the mixture in a heated inclined revolving drum devoid of projections on its interior,

and maintaining the temperature of the drum so that the heated material leaves the drum at a temperature of not less than 300 F. and not more than 350 F., adding to the heated material a reconditioning material such as bitumen and a filler in such quantities that the finished. product when heated and compressed in a manner which will reproduce the compression obtained when the mixture is laid on the road shall have 1.5% to 5% of voids and shall have a resistance to deformation of approximately between 1,500 and 3,000 pounds, the said resistance being expressed by the load in pounds which, when applied over an area 2 inches in diameter centrally situated on the upper surface of a disc four and a half inches in diameter and one and a half inches thick, the said disc having been formed of the material under examination at a temperature of 300 F. and under a pressure of 5,000 lbs. per square inch and maintained at a temperature approximating to the maximum temperature experienced on sun exposed surfaces in the country in which the mixture is to be laid for a period of one hour prior to testing, shall cause the peripheral ring or" uncompressed material to fracture.

4. Process of reconditioning used asphalt paving material consisting in increasing the voids in a heated and compressed sample of the pulverized material to aminimum of seven and a half per cent. by the addition of mineral material, heating the mixture in a heated inclined revolving drum devoid of projections on its interior, maintaining the temperature of the drum so that the heated material leaves the drum at a temperature of not less than 300 F. and not more than 350 F. and conveying the hot combustion gases which heat the exterior of the drum into direct contact with the material under treatment, adding to the heated material a reconditioning material such as bitumen and a filler in such quantities that the finished product when heated and compressed in a manner which will reproduce the compression obtained when the mixture is laid on the road shall have 1.5% to 5% of voids and shall have a resistance to deformation of approximately between 1,500 and 3,000 pounds, the said resistance being expressed by the load in pounds which, when aplid p'erature approximating to the maximum temperature experienced on sun exposed surfaces in the country in which the mixture is to be laid for a period of one hour prior to testing, shall cause the peripheral ring of uncompressed material to fracture. d p

NORMAN HENRY TAYLOR. 

